Fixed anchorage for concrete prestressing method with subsequent attachment



Feb. 4, 1.969

K. BUYER 3,425,177 FIXED ANCHORAGE FOR CONCRETE PRESTHESSING METHOD WITH SUBSEQUENT ATTACHMENT Filed Sept. 23, 1966 Inventor: A4425 UY United States Patent 3,425,177 FIXED ANCHORAGE FOR CONCRETE PRE- STRESSING METHOD WITH SUBSEQUENT ATTACHMENT Karl Buyer, Grafelfing, Bavaria, Germany, assignor to Messrs. Heilmann & Littman Bau-Aktiengesellschaft, Munich, Germany, a corporation of Germany Filed Sept. 23, 1966, Ser. No. 581,491 U.S. Cl. 52-230 3 Claims Int. Cl. E04c 5/08; E04g 21/12 ABSTRACT OF THE DISCLOSURE The invention relates to a fixed anchorage for stressed members consisting of several individual stressed pieces of steel, particularly stressed pieces of cold-drawn steel, for use in concrete prestressing with subsequent attachment.

For the fixed anchorage of such stressed members, i.e. for the anchorage of that end of the stressed member which is not to be connected to the stressing apparatus, the aim is to use the simplest possible means with low cost. To this purpose, so-called fan anchorages have become known in which the ends of the stressed wires diverge at the fixed anchorage end in a fan-like manner and are embedded directly into the structure of the concrete. The fan-like divergence is intended at the same time to provide a certain wedge effect.

These known fan anchorages, in which the ends of the stressed wires are embedded directly into the concrete of the structure, have the disadvantage that it is diflicult under the conditions on site to make them of a quality which will ensure the necessary anchorage strength. The cleavage tensile reinforcement necessary to take up the cleavage tensile forces in the region of the force transmission and the concentration of the stressed wires diverging fan-like at the end of the stressed member on the adjacent stressed-member cable mean that relatively densely reinforced parts are formed directly at the entrance into the stressed-member cable; in this connection, experience has shown that it is practically impossible to effect a satisfactory introduction of the building concrete, which as is known contains grain sizes of up to 30 mm. and in which the stressed-member anchorage is embedded directly. This results in a concrete of uncertain and frequently reduced quality precisely at the point of introduction, i.e. at the point where the bond stress is highest, and this concrete is unable to withstand the enormous stress. The introduction of the concrete into these known fan anchorages is particularly difficult in the case of stressed members disposed at the bottom, as a result of the poor accessibility, especially when compactors (vibrators) are used. A further disadvantage is that the arrangement of the individual stressed wires necessary to obtain a satisfactory foroe transmission, that is an arrangement which is as radially symmetrical as possible, is liable to be lost during the concreting operation.

A fixed anchorage has already been proposed in which to overcome the disadvantages outlined the stressed member is provided at its fixed anchorage end with a pretit) ice

fabricated anchor head consisting of an anchor sleeve which surrounds preferably coaxially the ends of the stressed wires over their embedded length and in which said ends are embedded by means of a special concrete of particular quality. In this known arrangement, the anchor sleeve surrounding the ends of the stressed wires is in the form of a smooth steel tube; within this tube, the ends of the stressed wires diverge in fan-like manner from a point at the inner end face of the anchor head where the bundle has a minimum diameter to a point at the outer end face thereof where the bundle has a maximum diameter.

By prefabricating the anchor head, this known anchorage admittedly obviates the working difliculties mentioned of fan anchorages in which the ends of the stressed wires are embedded directly in the building concrete. However, since in this known construction the tubular sleeve is in the form of a smooth tube, it can at the most act as cleavage tensile reinforcement in its axial direction; but it in no way provides a transmission of the anchoring force to the surrounding concrete along the entire anchor head length; on the contrary, the transmission of the anchoring force takes place solely at the relatively small inner end face of the anchor head. The known construction thus has pronounced disadvantages as regards the force transmission, which is a decisive factor in the reliability of such an anchorage: Because the force is transmitted practically solely in the region of the inner end face of the anchor head, both the sealing concrete and the adjacent building concrete are overstressed at this point, since maximum values of the cleavage forces and the bond stresses between the individual stressed wires and the sealing concrete also arise at the said end face. To obtain an anchorage which is at all usable with the known design, the tubular sleeve would have to be of extremely r great thickness to withstand the strain at the end face,

since as already mentioned the concrete at the inner end face must take up the entire anchoring force and also the highest values of the cleavage forces and the bond stresses between the steel wires and the sealing concrete. Even if the tubular sleeve was made of suflicient thickness, which would be unpractically high, this would not overcome the fundamental defect of the known construction that the transmission of the anchoring force from the anchor head to the building concrete is limited almost exclusively to the end face of the anchor head, producing undesirable stresses at this point. The length of anchor sleeve necessary to secure the ends of the stressed wire in the anchor head play in the known construction practically no part in the force transmission from the anchor head to the building concrete. In addition to the undesirable concentration of the load in the region of the end face of the anchor head, this represents a further considerable disadvantage in that it is basically desirable in prestressed concrete structures for the force transmission to take place as far as possible towards the outside at the beginning of the structure, so that as high a fraction as possible of the latter may be assembled in the prestressed state. However, in the known construction the force transmission takes place almost completely at the inner end face of the anchor head and thus at a :point at a relatively great distance inside the structure.

The invention thus relates to a fixed anchorage for stressed members consisting of several individual stressed pieces of steel, particularly stressed pieces of cold-drawn steel, for use in concrete prestressing with subsequent attachment, in which the stressed member is provided at its fixed anchorage end with a prefabricated anchor head consisting of an anchor sleeve which surrounds preferably coaxially the ends of the stressed wires over the embedded length and in which the ends of the stressed wires are embedded by means of a special sealing concrete of particular quality. The object of the invention is to obviate the outlined disadvantages of the known fixed anchorage and to obtain an optimum force transmission from the anchor head to the building concrete.

For this purpose, according to the invention the anchor sleeve is provided with a profilation to obtain an effective force transmission into the building concrete over the entire axial length of said sleeve; the anchor sleeve is preferable constructed as a profiled anchor head made from closely wound round steel or from wound sheet metal.

This simple measure according to the invention achieves a forces transmission from the anchor head into the building concrete over substantially the entire axial length of the anchor sleeve; the force transmission is thus not limited to or concentrated on the end face, as in the known construction. On the contrary, in the construction according to the invention the transmission of the anchoring force takes place continuously over the entire length of the anchor head, so that the entire embedded length of the ends of the stressed wires within the sealing concrete of the anchor sleeve is employed at the same time for the transmission of the forces from the anchor head to the building concrete. This meets the general requirement in prestrcssed-concrete structures that the force transmission takes place as near as possible to the outside of the structure. The construction according to the invention obviates the undesirably high load concentrations which necessarily arise in the known construction in the region of the inner end face of the anchor head. With the construction according to the invention, the anchor sleeve itself can be constructed relatively thin; when the anchor sleeve according to the invention is made in the form of a profiled sheet-metal pot, for example, the latter acts in the finished state, i.e. after attachment to the building concrete, practically only as medium between the sealing concrete and the surrounding concrete of the building.

Providing the fixed anchorage ends of the stressed members away from the concrete of the structure with a prefabricated anchor head according to the invention, which may be made of higher quality and under optimum conditions, particularly as regards the quality of the sealing concrete in which the ends of the stressed wires are to be directly embedded, ensures a reliable fixed anchorage and at the same time a considerably reduced embedded length as compared with the known anchorages. The reduction is not only especially desirable because it facilitates handling, but also because of the better and more clearly defined force-transmission conditions. The possibility afforded by the invention of putting in place in the building a completely finished fixed anchorage represents a considerable advance.

According to a particularly advantageous form of embodiment of the invention, the anchor sleeve is sealed at its inner end face by means of a perforated disc which comprises several radially symmetrical holes, through which the stressed wires enter the anchor head, being thus held in radially symmetrical arrangement if desired, means in the form of an annular perforated disc for holding the stressed wires in radial symmetry may also be provided at the outer end face of the anchor sleeve.

As a result, it is ensured that the individual stressed wires are held in radial symmetry.

The ends of the stressed wires may preferably diverge fan-like within the anchor sleeve in known manner, but they may be also unspread, i.e. extend substantially parallel to the axis in the anchor sleeve.

To increase the bond strength, the ends of the stresses wires may be corrugated in known manner over their embedded length within the anchor sleeve.

According to preferred forms of embodiment of the invention the anchor sleeve may be made in the form of a helix wound from round steel or in the form of a pot wound from sheet metal with outer profiling; it is essential that the profiling of the anchor sleeve is sufiicient to ensure a good bond between the concrete seal and the surrounding building concrete this is ensured to a sufficient degree by both the construction in the form of a helix wound from round steel and by that in the form of a pot wound from sheet metal; in the case of the construction in the form of a sheet-metal pot, the latter acts in the finished state, i.e. after attachment, practically only as medium between the sealing concrete and the surrounding building concrete.

The special sealing concrete used to fill the anchor sleeve and to embed the ends of the stressed wires preferably contains substantially only grain sizes of up to 2 mm.

It is of course preferable to provide an outer cleavage tensile reinforcement in the region of the fixed anchorage, which may be constructed in conventional manner in the form of a helix of round steel surrounding the anchorage end.

Further advantages and details of the invention will be apparent from the following description of one example of embodiment made with reference to the drawing, which shows a longitudinal section through the fixed anchorage end of a stressed member according to the invention.

The stressed member, denoted as a whole by 1, consists of several pieces of stressed wire or of stressed steel preferably arranged in radial symmetry about the longitudinal axis AA, two of which are shown in the drawing in section at 2 and 3 and others of which are shown diagrammatically in side view. The individual stressed wires of the stressed member extend for the major part of their length (only a small fraction of which is illustrated in the right half of the figure) parallel to the axis within a sleeve 4 surrounding the stressed member 1 relatively closely.

The stressed member is provided at its end which is illustrated in the drawing and which serves for the fixed anchorage with the anchor head according to the invention, denoted as a whole by 5 and prefabricated on the end of the stressed member outside the structure and before the stressed member is put in place in the latter.

This anchor head consists of an anchor sleeve or anchor pot 6 which surrounds the end of the stressed member over the embedded length thereof and which has a diameter substantially greater than the normal diameter of the stressed member and its sleeve 4. In the anchor sleeve, the stressed wire ends diverge preferably in the manner of a fan, although they preferably continue to run in radial symmetry, as shown in the drawing by the ends 2a and 3a of the two stressed wires shown in section 2 and 3 respectively.

In the example of the embodiment illustrated, the anchor sleeve or pot 6 is made in the form of a anchor spiral wound from standard steel wire; however, it would be possible to construct the anchor sleeve from sheet metal.

The stressed wire ends are preferably corrugated in a manner known per se along that part of their length which extends within the anchor head in order to increase the 'bond strength and to obtain sufficient anchorage strength, especially in the case of completely smooth stressed pieces of cold-drawn steel, for which the anchorage according to the invention is preferably provided.

The anchor sleeve 6 is closed at its inner end face by a perforated disc 7 which is preferably cup-shaped with a peripheral flange 8 which bears against the outside of the anchor spiral. The perforated disc 7 is provided with a number of radially symmetrical holes 9, 10 corresponding to the number of stressed wires, the latter entering the head through said holes and being thus held firmly in radially symmetrical arrangement.

Preferably, a closure plate or closure cup 11 having a large central opening in the base and a number of holes corresponding to the number of stressed wires may also be provided at the outer end face of the anchor head, so that the wires are held in radially symmetrical arrangement at their outermost ends as well.

Provided at the inner end face of the anchor head, adjacent the perforated disc 7, is a transition pipe 12 with the filler pipe 13. The fan-like divergence of the stressed wire starts within the transition pipe 12. The filler pipe 13 serves for the injection of the mortar after stressing, producing the subsequent attachment between the stressed member and the surrounding concrete of the building.

According to the invention, the anchor sleeve 6 is filled with a specially prepared sealing concrete 15 of high quality, which preferably contains no grains of over 2 mm. size, ensuring that the quality can stand up to the forces transmitted at this point, where the stress is highest.

Providing the fixed anchorage end of the stressed member with the prefabricated stress head according to the invention ensures a reliable anchorage and force transmission, requiring to do this only a relatively short penetration length, for example only half that required with the hitherto usual fan anchorages. In the example of embodiment illustrated, the length of the anchor spiral is for example only 25 cm., where as in the otherwise usual fan anchorages penetration lengths of up to 1 m. are necessary.

14 denotes the usual cleavage tension reinforcement in the form of a helix of standard steel wire which surrounds, preferably coaxially, the anchor head 6 and the adjoining parts 12, 4 of the stressed member. This tension reinforcement serves in known manner to take up the cleavage tensile forces and to provide uniform transmission of the anchoring force in the region of the anchor head into the concrete of the structure.

The fixed anchorage according to the invention has the advantage of prefabrication and can be made away from the building under optimum conditions, so that it may be completely finished before being put int-o place in the structure. To illustrate a typical example, the fixed anchorage of the example of embodiment described may have the following dimensions and contain the following materials: The stressed member may for example consist of twelve individual stressed wires 7.5 mm. in diameter and made from St 150/ 170 cold drawn steel; the corrugations of the ends of the stressed wires may have a wave length of 50 mm. and a wave height of 4 mm; the anchor spiral may be closely wound from St I steel wire mm. in diameter, with a spiral diameter of approx. 15 cm. Due to the gOOCl bond and excellent force transmission achieved with the invention, a value of for example only 25 cm. approx. is suflicient for the axial length of the anchor spiral. The tensile reinforcements may be wound in the fonm of a helix from St I steel 10 mm. in diameter with a pitch of 50 mm.

As already mentioned, the anchor sleeve 6 may be made in the form of a pot wound from sheet metal instead of a helix wound from steel wire; in both cases, sufiicient profiling is required, and is provided by both the construction in the form of a steelwire helix and that in the form of a pot wound from sheet metal.

The dimensions and details of the example of embodiment given serve of course only to explain the invention and are not intended to be limitative. The fixed anchorage according to the invention is basically suited to all types of stressed members having several stressed wires of any dimensions.

What I claim is:

1. Fixed anchorage for stressed members for use in concrete prestressing with subsequent attachment;

said stressed members consisting of a plurality of individual stressed steel wire elements; said stressed members each having a fixed anchorage end section;

at said end sections, said stressed members are provided with a prefabricated anchor head; said end sections being corrugated along their respective lengths;

said head comprising an anchor sleeve which surrounds said anchorage end sections of said stressed members coaxially over their length within said sleeve; said sleeve being comprised of closely wound metal windings extending along said sleeve with each winding abutting its neighboring winding and forming an exterior corrugated profile for said sleeve, whereby each corrugation of said profile is able to engage the concrete surrounding said sleeve to cause an efiective force transmission into the concrete surrounding said sleeve over the entire axial length of said sleeve;

eaid head having an inner face; a first perforated disc over said inner face; said first disc having a plurality of radially symmetric perforations, through which said stressed members enter said head and are positioned therein in a radially symmetric arrangement;

said head having an outer face; a second perforated disc over said outer face and having a plurality of perforations through which said stressed members pass and by means of which said stressed members are held in a radially symmetric arrangement;

said stressed members diverge fan-like from said inner to said outer anchor face and said perforations in said first and said second disc are located to maintain this divengence;

each of said first and said second discs having an annular flange secured to it; each said flange extends inward over the portions of said windings near said first disc and said second disc, respectively; said flanges covering the said corrugations which they surround;

said anchor sleeve having sealing concrete in it within which said anchorage end sections of said stressed members are embedded; said concrete contains substantially only grains of sizes up to 2 mm.

2. Fixed anchorage according to claim 1, wherein said anchor sleece is formed as a closely wound coil of round steel.

3. Fixed anchorage according to claim 1, wherein an outer cleavage tensile reinforcement means is provided in the region of the fixed anchorage.

References Cited UNITED STATES PATENTS 2,618,147 11/1952 Freyssinet 52230 2,751,660 6/1956 Nakonz 52230 X 2,934,935 5/1960 Zerna et al. 52223 2,970,406 2/ 1961 FinsterWalder 52223 3,300,922 1/ 1967 Buyer 52230 FOREIGN PATENTS 721,517 l/1955 Great Britain. 81,804 6/ 1956 Netherlands.

OTHER REFERENCES 1,154,609, 9/1963, German Printed Application, Boswau et al. 52-230.

1,168,046, 4/1964, German Printed Application, Leonhardt, 52-230.

1,193,226, 5/1965, German Printed application, Heilmann et al., 52-230.

ALFRED C. PERHAM, Primary Examiner. 

